The present invention relates to a compact control device for failsafely controlling an electrical actuator, in particular for controlling a safety brake as is used, for example, in elevators and other movement axis affected by gravity. Furthermore, the invention relates to an apparatus for the failsafe holding of weights, such as an elevator or the like, comprising a mechanical brake and a compact control device.
It is known to use safety brakes in order to safeguard elevators and other vertically raised weights against uncontrolled falling. DE 10 2005 048 208 A1, for example, describes such a safety brake for elevators. The safety brake has a rotor, which is connected to a shaft to be braked in rotationally fixed fashion, and one or more armature plates, which secure the rotor via a friction lining. The armature plates are biased via springs such that they prevent a rotary movement of the rotor in the rest state. In order to release the brake, an electromagnet needs to be excited with a control current in such a way that the armature plates are pulled away as a result of the magnetic force of the rotor. As soon as the current is interrupted, the armature plates press against the rotor again as a result of the spring force, with the result that the weight is safely stopped in the event of a power failure.
Switching-on and especially switching-off the current by the electromagnet is a safety-critical task since an undesired current flow, for example as a result of a fault in the electrical control system, can cause undesired opening of the brake and therefore falling of the weight. It is therefore common practice to control the electromagnet using a failsafe PLC in accordance with relevant standards for machine safety. A suitable safety controller generally needs to meet the requirements according to SIL 3 of the European Standard EN IEC 61508 and/or in accordance with PL e of the standard EN ISO 13849. Such a safety PLC ensures, by virtue of redundancy, regular internal function tests and/or further measures, that a control output does not cause an undesired current flow even when a fault, for example a short circuit, occurs in the safety PLC and/or in the connecting lines to the brake.
Furthermore, it is known to monitor the state of the safety brake. For example, the safety brake known from DE 10 2005 048 208 A1 has a microswitch, which signals the so-called application and release of the brake, i.e. the release of the armature plates from the rotor with the aid of the electromagnet. However, the outputs of conventional safety PCs are not designed to provide the high control current required for releasing an elevator brake. In practice, the control current is therefore switched on and off by means of contactors, wherein the contactors are actuated via the outputs of a safety PLC. This necessitates that the safety PLC monitors the functioning of the switching contactors in order to eliminate the possibility of an undesired current flow through the electromagnet of the safety brake. Furthermore, the safety PLC can monitor the state of the brake by means of the microswitch. All of the control and monitoring functions need to be programmed in the safety PLC and validated by the constructor of the installation.
DE 103 25 363 A1 describes a safety switching device for the failsafe disconnection of an inductive load, for example in the form of contactors or solenoid valves. During disconnection of the current through such load, an overvoltage, which is often limited by means of a suppressor diode, is generated as a result of mutual induction. However, the time required for the disconnection is longer the more the overvoltage is limited. The safety switching device of DE 103 25 363 A1 therefore has an anti-surge element, which has a first and at least one second anti-surge operating mode, which can be activated optionally with respect to one another, wherein the limitation of the overvoltage is different in the two anti-surge operating modes. Therefore, this safety switching device can optionally effect quicker, abrupt disconnection at a high induced voltage or slower, soft disconnection at a low induced voltage.